Carbon-carbon composites are important materials for aerospace and other applications which require high strength and toughness at high temperature. The use of carbon-carbon materials are limited because of their susceptibility to oxidation, particularly at elevated temperatures. Oxidation protection may be provided by coating the carbon-carbon composite with an oxidation resistant metal such as e.g., aluminum. Aluminum is desirable because of its high electrical conductivity, high thermal conductivity and light weight characteristics. Although processes for bonding aluminum to graphite have been developed whereby aluminum is chemically bonded to carbon or graphite by the formation of Al.sub.4 C.sub.3 ; such bondings are susceptible to humid conditions and readily peel or flake off. The low thermal expansion characteristic of carbon-carbon and its low surface reactivity make it extremely difficult to apply a uniform coating of aluminum to a carbon-carbon material which will chemically adhere to the carbon-carbon surface without distortion and without cracking or delaminating in response to thermal swings.
An improved method for forming a chemically bonded aluminum coating to a carbonaceous article with a metal carbide interface is disclosed in U.S. Pat. Nos. 4,104,417; 4,347,083 and 4,252,856 respectively. In accordance with the teachings of these patents titanium, hafnium or tantalum may be used to bond aluminum to carbon via an intermediary bond or intermetallic phase, formed in situ, of titanium carbide, hafnium carbide or tantalum carbide. A tantalum-aluminum containing mixture or slurry is deposited on a carbon substrate and the carbon substrate is then heated in an inert atmosphere to form a thin aluminum outercoat chemically bonded to the carbon substrate via a metal carbide interface of tantalum carbide. This process produces excellent results for a carbonaceous substrate of carbon or graphite other than carbon-carbon. When applied to a carbon-carbon composite the coating spalls and flakes off and the aluminum distorts.